Universally, waterjet propulsion systems are mounted on the main stern transoms of surface vessels with at least a portion of the pump and the pump exit nozzle above the surface of the water. That location permits the actuators for the steering nozzle and reversing deflectors of the propulsion system to be above the water, thus simplifying the installation and maintenance of the actuators and the hydraulic lines leading to the actuators. Also, it is common to provide access ports in the pump above the waterline to permit the pump to be serviced without drydocking the vessel.
Generally, the intake opening to the water supply conduit for the waterjet pump is located on the bottom of the hull a short distance forward of the transom and just far enough below the waterline to ensure that water is taken in under most operating conditions of the vessel--i.e., absent very severe pitching of the vessel due to heavy waves when intake may be briefly interrupted by surfacing of the opening. The location of the intake opening at a minimum height below the pump improves efficiency, as compared to a deeper location, by minimizing the vertical distance that the pump has to pump the water from the intake opening to the pump rotor.
A disadvantage of having the waterjet pump relatively close to the water surface is the reduced hydraulic head of water at the pump inlet. The reduced suction head reduces the capability of the pump to absorb high power at slow speeds due to the onset of cavitation. The pump has to be larger than it would have to be if the suction head were greater in order to provide high power output at slow speeds without cavitation.
Another disadvantage of previously known waterjet propulsion systems is the relative complexity of the actuators for the steering nozzle and the reversing deflectors and the outboard location of the actuators. The actuators are usually hydraulic piston/cylinders and require hydraulic lines that penetrate the hull. In the event of leakage, the hydraulic fluid contaminates the environment. The outboard actuators and the lines that serve them are vulnerable to damage from impacts.